During the follow-up period researchers recorded 602 deaths – 1.4% of the total group. These included 219 deaths caused by cancer and 34 by heart and artery disease.

The scientists, led by Dr Laure Schnabel from Paris-Sorbonne University, wrote in the journal Jama Internal Medicine: “An increase in ultra-processed foods consumption appears to be associated with an overall higher mortality risk among this adult population.

“Further prospective studies are needed to confirm these findings and to disentangle the various mechanisms by which ultra-processed foods may affect health.”

Consumption of ready meals and other forms of ultra-processed food accounted for an average 29% of total calorie intake, the study found.

It was also associated with younger age, poorer education, living alone, lower physical activity and higher body weight.

These and other factors were taken into account by the researchers.

There could be several explanations for the findings, the scientists said.

Some ultra-processed foods contained large amounts of salt, and high sodium intake had been associated with higher rates of cardiovascular disease death and stomach cancer.

Excessive sugar in the diet was also linked to a greater risk of cardiovascular disease death.

In addition ultra-processed foods contained little fibre, which had been shown to reduce mortality risk.

Food additives, frequently found in ultra-processed foods, may also play a role, said the researchers.

For instance, titanium dioxide was widely used by the food industry but had been linked to intestinal inflammation and cancer.

There was also evidence that consumption of emulsifiers could have an effect on gut bacteria and promote cancer and metabolic syndrome, a pre-diabetic condition.

British statistician Professor Kevin McConway, from the Open University, warned that the French findings may not necessarily apply to the UK.

He said: “People’s diets in the UK are, on average, very different from those in France. In the UK we (on average) eat far more in the way of foods classified as ultra-processed, but we also probably eat different types of ultra-processed foods, and indeed different types of foods that aren’t ultra-processed.

“Since this study doesn’t give any data on what aspects of the foods might cause problems, it could be that its findings don’t carry over to the UK at all – we just can’t tell.”

Professor Nita Forouhi, from Cambridge University, said: “The case against highly processed foods is mounting up, with this study adding importantly to a growing body of evidence on the health harms of ultra-processed foods.

“Though this research is observational, which has well-known limitations, the authors have made demonstrable attempts to reduce these. But some important limitations remain.”